Exhaust gas recirculation (EGR) for internal combustion engines is known. Typical compression ignition engines may also cool recirculated exhaust gas in EGR coolers. In an engine that is not naturally aspirated, for example, an engine having a turbocharger, a difference in pressure between the exhaust system and intake system of the engine may determine a maximum amount of exhaust gas that may be recirculated from the exhaust into the intake of the engine.
Exhaust gas in the exhaust system upstream of the turbocharger is at an exhaust pressure, or exhaust back pressure (EBP) during operation of the engine. Conversely, air or a mixture of air and exhaust gas in the intake manifold of the engine is at an intake manifold pressure, or a manifold absolute pressure (MAP) during operation of the engine. An EGR valve is usually employed to fluidly connect the exhaust and intake manifolds. When the EGR valve is opened, exhaust gas flows from the exhaust system into the intake system of the engine. Primary factors that determine a capacity of the EGR system on an engine to flow exhaust gas are the size of the EGR valve opening, and a difference of pressure between EBP and MAP, typically referred to as Delta P.
Some engines may require more EGR gas than what the engine is able to provide during operation, partly because many engines are advantageously designed to run under low Delta P conditions because such conditions are conducive to high fuel economy. Nevertheless, even under conditions of low Delta P, demands for EGR gas flow increase as emissions requirements for the engine decrease.
There have been many methods to augment EGR gas flow on an engine having low Delta P during operation. One method is to use an intake throttle valve configured to constrict air flow into the intake manifold of an engine, placed at a location upstream of a point of mixing of exhaust gas and air in the intake system. By closing the intake throttle valve, the MAP is lowered and Delta P will increase. One disadvantage of this method is that pumping loss in the engine increases thus lowering fuel economy and the power output of the engine.
Accordingly, there is a need for augmentation of EGR gas flow in an engine having a turbocharger that does not depend solely on use of an intake throttle valve or other methods.